Testing ophthalmic lenses



pril 19, 1938. A, AMES., JR, 'ET-AL 2,114,282

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TESTING OPHTHALIVIIC LENSES 12 Shets-Sheer. 5

Filed Jan. 25, 1936 A. AMES, JR., ET AL, 2,114,282

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TESTING OPHTHALMIC LENSES Filed Jan. 25, 1936 l2 Sheets-Sheet lO pril19, 1938.

A. AMES, JR., ET AL TESTING OPHTHALMIC LENSES Filed Jan. 25, 1936 l2Sheets-Sheet ll April 19, 1938. A. AMES, JR., ET A1. 2,114,282

TESTING OHTHALMIC LENSES Filed Jan. 25, 1936 l2 SheeS-Sheei'l l2 f5 m5f' Patented Apr. 19, 1938 TESTING OPHTHALMIC LENSES Adelbert -Ames, Jr.,and Kenneth N. Ogle, Hanover N. H., assignors to Trustees of DartmouthCollege, Hanover, N. H., a corporation of New Hampshire ApplicationJanuary 25, 193e, semi No. casas` 17 Claims.

It is in certain cases desirable or even necessary exactly to determineby experiment the optical eiect of lenses in order to check whether ornot they actually perform the functions for which they have beendesigned and ground. For example, lenses or lens combinations describedin Patent No. 1,933,578, of November 7, 1933, to Adelbert Ames, Jr., andcopending application Serial No. 750,162, filed October 26, 1934, de-

0 signed from data obtained with the testing methods and instrumentsdescribed for example in Patent No. 1,944,871 of January 30, 1934, toAdelbert Ames, Jr. and Gordon H. Gliddon, are often rather complex sincethey are to correct not only ordinary spherical and astigmaticrefractory defects, but also ocular image incongruities (also calledaniseikonia) and may, moreover, have prismatic components. Since themanufacture of such lenses is difficult, they will not always as amatter of fact exactly comply with the prescription and the clinicalexamination data; since further successful elimination of the patientstroubles depends on the exact compliance with the recommendationsderived from the clinical examination as expressed in the examinationdata and the prescription derived therefrom, it is imperative to checkthe finished lenses to see whether or not they will have the desiredeffect on the eyes of the person who is to wear them.

The present invention has the principal object of providing a method formeasuring the optical eect of a lens or lens system upon an eye, andinstrumentalities for carrying out this method.

'I'he problem here involved is quite different from that of measuringthe distortion of, for example, a photographic lens. This is due to thefact that in the present case the effective displacement of the lightrays, produced by lenses placed before the eye, that is at or near itsanterior focus, has to be determined. There are thus introduced into theproblem, among others, the complicating factors of the distance of thelens or the lenses from the nodal points of the eye,-the presence of arelatively small pupil at a d considerable distance from the lenses andthe desirability of considering the convergence of the eyes implyingvarying positions of the eye relative to the lens. in question have solittle power that their own J nodal points are usually at veryconsiderable distances from the lenses.

As indicated above, the new method involves changes in size and shape ofthe optical image produced by lenses, alone or together with dioptricpower effects, and not simply image distortions as that term isordinarily used, that is, meaning a measure of the variation ofmagnification of the image as the light rays move away from the axis.These distortion effects in the ordinary meaning of the word are, ofcourse, also Moreover, many of the lenses to be measured, but thepresent method is primarily concerned with image magnitude changes,symmetrical to a point or a line or asymmetrical, and in additionprismatic eifects. All these changes of the incident light rays willherein be referred to as optical lens effect or simply effect of thelens.

In one aspect, the new method contemplates the reproduction of theoptical function of the eye with and without an effective lens combinedwith the eye, in a manner permitting the exact comparison of the twosituations and also permitting consideration of relative movementbetween eye and lens.

In another aspect, the new method contemplates the determination of thepaths of light rays between a light source and a reference point xed inspace, with and without an interposed optical system, and the comparisonbetween the two paths, namely the path with and the one withoutinterposed system. The distance between reference point and opticalsystem may be variable in order to measure diierent types ofmagnification.

In still another aspect, the new method proposes to reproduce, and todetermine the optical cooperation of an eye and an eyeglass, bydetermining the path of only such light rays through the systemeyeglass, that pass through a point of the eyewhich is so selected thatthe dioptrically eiective elements of the eye can be omitted from thesystem.

It is also a feature of the present invention to measure the dioptricpower of the lens system to be tested by focusing a telescope on a fixedlight source through the lens system, or by moving the light sourceuntil it is sharply in focus, or by combining both procedures. In apreferred embodiment, the light rays used for detecting the effect of alens system travel in the natural direction, that is from their sourcetowards the front surface oi' the lens system leavin-g it at its ocularsurface. In another embodiment, .this path may be reversed, the lighttraveling in a direction opposite to that in which it would pass thespectacle lenses during actual use. In the first instance, the apparatuswhich selects a. light ray, as for example a telescope which may also beused for determining the lens power, is on one side of the structurerepresenting the eye, the light source and the lens to be tested beingon the other side; in the second instance both telescope and lens to betested are on the same side of that structure. Provisions are also madefor independently focusing in both principal axes of lens systems withcylindrical elements, and for aligning, and measuring the inclinationsof such axes.

The method according to our invention contemplates not only, as abovementioned, tests lens relatively to the eye, but it also considersdifferent visual distances, so that, for example, the usual readingposition with the eyes looking downwardly and converging at a distanceof about 40 cm. can be reproduced as well as relaxed vision with theeyes looking straight ahead at practically infinite distance.

Further features of the invention are instruments for carrying out theabove-described method, which instruments provide for passing light raysthrough a lens to be tested and a lxed reference point, and for exactlyand conveniently determining the paths oi the rays as affected byvarious positions of the lens relatively thereto.

These and other features. objects and aspects will be apparent from thedetailed explanation of the genus of the invention with referencetoseveral concrete embodiments thereof. description refers to drawingsin which:

Figs. l, le and ib are diagrams illustrating the principle of theinvention; Figs. 2, 3, fie, Lib and 10, similar diagrams plainingseveral modiiications or the invention' Fig. 5 is a diagrammaticisometric view oi instrument especially suited for carrying out the vmethod illustrated in Fig. with the proportions of the instrumentsomewhat distorted order better to demonstrate its operation;

Fig. 6 is a side elevation oi' the instrument schematically shown inFig. 5;

Fig. '7 is a plan view oi that instrument;

Fig. 8 is a section on lines o Fig. 6; Fig. 9 is section through theadjusting mechanisms between instrument base and .telescope frame;

Fig. 10 is a section on lines iii-l@ of Fig. 9;

Fig. 11 is a side elevation of the telescopes and their supports, seenfrom the right hand sides of Figs. 6 and '7;

Fig. l2 is a side elevation of the support or" the aligning telescopeshown in Fig. li;

Fig. 13 is a section on lines lS-l o Fig. l2; Fig. 14 is a section onlines lf3- itl of Fig. 8; Fig. 15 is a section on lines l5-i5 of Fig. 8;Fig. 16 is a section on lines iii-i6 of Fig. l5; Fig. 17 is a section onlines il--il of Fig. 6; Fig. 18 is a section on lines id--li of. FLg.i7; Fig. 19 is a plan view of the lens support;

Fig. 20 is a section on lines @@-Ed of Fig. 17; Fig. 21 is a section onlines Ei-Z of Fig. 17; Fig. 22 an elevation of the lens adjuster shownin Fig. 6;

Figs. 23 and 25 diagrammatical representations of devices for focusinglenses with cylindrical elements; and

Figs. 24 and 26 diagrams illustrating the aligning of lenses withcylindrical elements.

The principles of the invention as governed by the conditions prevailingduring the correction of ocular defects with lenses placed before theeyes will rst be explained with reference to Fig. 1. In this gure, E isan eye with center of rotation R, pupil P, and anterior focal plane F.As mentioned above, a. reference point may be selected with a view toeliminating from the entire system the optical effect of the eye. Forthat purpose, and in order to obtain the total magnication effect of thelens to be tested, a pseudo nodal point N is chosen, as will beexplained more in detail hereinafter. For measuring other types ofmagnification, different reference points may be'used.

Placed with its ocular surface approximatelyy at F is shown a lens L tobe tested, of any de- Aconsidering different positions of the spectaclesired shape, and schematically indicated as a plate. Assuming first thatlens L is not placed before the eye, rays u, o, w are principal raysfrom points U, V, W of a field through point N to points U1, V1, Wi onthe retina. When the lens is placed before the eye, rays to the sameretinal points are deviated by lens L to positions w', o', coming frompoints il" and t of. the `field. This means that, with the lens inplace, points if" and W will appear to be at the same pieces or theobject ield as points 'i7 and W seen without lens. Assuming for examplethat L is e.

curved plate with parallel surfaces, concave toward the eye, a; t-.fouldbe deviated as indicated in Fig. l.; that a field oi smaller angularsize would appear the saine size on the retina as v a larger angularfield before interpositon or the lens, the latter enlarging the field.

It is the aim of the present invention to measvure the deviations, by alens, of rays coming from various points of the held, or, in otherwords, to measure the amount of apparent displacement of points in theeld caused by lenses interposed between eld and eye.

This could be accomplished byv placing behind the lens a number oftelescopes directed at a reference point, for example pseudo nodal pointN, and rotatable about that point. Such telescopes are indicated at TU,TV and TW o Fig. l. The telescopes may be adjusted to image points U, Vand W sharply on the cross hairs of the respective telescopes, with thelens to be measured not in place. If that lens is then put in its properrelation to N. the images oi the previously selected object points Willdeviate more or lessfrom the respective cross hairs. The amount of.deviation can then be measured by determining the arc through which thetelescopes must ce rotated to bring points U, V, W back to the crosshairs. The relative position of eye and lens are indicated in Fig. 1, asderived from so-called schematic eyes.

Although it would be quite `ieasible to employ several telescopes, forexample initially placed to point in the directions of selectedrepresentative rays, say in angular distances of 5, it is preferable touse a single telescope and to adjust the relative angular position or"lens and telescope, with respect to point N. Various arrangements of,this type are possible and will now be explained with reference to Figs.2, 3 and 4i.

in Fig. 2, N is again the nodal point representing an eye, and L a lensor lens system placed in proper relation to reference point N, asexplained with reference to Fig. 1. Telescope T is rotatable about N. Ifit is desired to investigate the lens properties at a visual angle a,the lens is placed in position as indicated, with its optical axispassing through point U of a scale S. The reading O is then taken, thelens removed, and a second reading I taken. The procedure is repeatedfor various angles a, the telescope being adjusted relatively to lensand scale, for each angle.

Elementary mathematics give the magnifications as follows for A0 smallin comparison with 0:

Angular magnification in =A Ge X 100 Arctan tively xed, whereas lens Lis movable relative thereto. It is supported in such a manner that itcan be lrotated about the reference point through predetermined anglesa. Readings I and O without, and with lens L respectively, are againtaken, and the magniilcations determinedas Angular magnification in ie9100= Arctan i' d :l: -i approximately :l: 55X 100 It will be understoodthat in Flg. 2, and I are read on scale S, whereas in Fig. 3, d is readon scale S and 0 on a protractor scale indicating the angular positionof the lens relatively to the telescope.

Fig. 4 shows a preferred embodiment where both lens and telescope aremoved in order to derive'the desired measurements. In this gure, Lisagain the lens to be tested, placed in a holder permitting its rotationabout reference point N through predetermined angles which may be readon scale Si. Between N and the lens is placed a diaphragm providing apupil, as indicated at P of Fig. 1. T is a telescope which, like lens L,can Abe rotated about point N, the amount of adjustment being read on ascale S2. A test object, preferably a point source of light, is arrangedat A.

With this embodiment of our testing equipment, the new method is carriedout as follows. With no lens in the lens holder, telescope T is soadjusted that scale S2 reads zero lwhen the image of light point A fallson the cross hair oi. telescope T. Lens L is then inserted in normalposition, that is, with its optical axis coinciding with the opticalaxis of the telescope. Assuming that lens L has no prism power, theimage of A will remain on the cross hairs of telescope T. The lens isthen rotated an angle a around N, a being the angle of the ray whosedeviation is to beA determined. Angle a can be derived from scale Si. Ifthe lens deviates the light ray, point A will be displaced from thecross hair. Telescope T is then moved until this image is again back onthe cross hair, the amount of adjustment being determined by means ofscale S2. Similar measurements are made with the lens in variouspositions at determined angles both sides o! the axis N-A.

The magnications can then be determined as:

Angular magnification in :teQX 100 tan A6 tan SI1-Han 9 tan AG] Thetesting method illustrated in Fig. 4 lends itself well to theapplication of various renements of procedure and also for being carriedout with the aid cf an instrument which is easy to handle butnevertheless yields very accurate results. This modification willtherefore be discussed more in detail, by referring to Fig. 4 and inaddition to Fig. which comprises all elements of Fig. 4, marked as inthat figure, but shown as incorporated in a diagrammatically indicatedinstrument which will be described more in detail hereinafter.

It was found that the eyes note, at the axis, ocular image sizedifferences as small as 2" to 5" Linear magnification in angularmagnitude, and at 5 from the axis about 30" or 0.2% angular sizedifference. Hence, it should be possible accurately to read 0.1% angulardeviation. This could be accomplished by means of scale S2 having avernier or reading microscope, andbeing directly associated with thetelescope T. It was found, however, that the procedure becomes lesstedious ii' a reading telescopepointing at a. distant scale is adopted.Fig. 5 shows aligning telescope TAwhich corresponds to telescope T ofFig. -4, and reading telescope TR arranged somewhat above TA and fixedthereto, so that both telescopes rotate about point N. Telescope TR.points at a scale ST2 which corresponds to scale S2 of Fig. 4. Simplemathematics shows that, in order to read 0.1% deviation at an angle a of5, and if one inch of scale ST2 is divided into 60 parts (which can beeasily distinguished), the distance between telescope TR and scale ST2must be 4851 mm. For the same distance, 30 divisions are necessary foran angle of and 20 divisions for an .angle of 15. Accordingly, sincemeasurements for these three angles are ordinarily sumoient, scale ST2comprises three component scales S5, SIO and SI5, shown in Fig. 5, eachindicating 0.1% deviation as one division thereof, for its particularangle a.

In order to permit speedy relative adjustment oi' lens L and telescopesTA and TR to these selected angles of 5, 10 and 15 degrees, scale Si ofFig. 4 is actually designed as a series oi' spring stops arranged atthese angles, indicated at SI of Fig. 5. Hence, if the spring stopcorresponding to a=5 is used, the effect of the lens to be tested isread in 0.1% on scale S5, and similar for the other angles oneither'side of the axis.

Since most lenses have a prism eilect, a fourth scale SP (Fig. 5) isadded which indicates, in prism diopters, the prism power of a testedsystem ior an a of zero.

It will. of course, be understood that more than three angles of anyselected magnitude can be used on either side of the axis, and that anactual conventional protractor scale can be employed instead of thespring stops.

Care must be taken in order to distinguish magniflcations anddiminutions. Appropriate markings on scales Si and ST2 will make thisdistinction easier. For example, if the sides of SI and ST2 are markedand for counterclockwise and clockwise rotation, respectively, amagnication is measured if both scales are used at the samev sign, and adiminution if the signs are opposite.

Since, as will now be evident, the effect of the lens to be testedchanges with the object distance, the latter must be taken intoconsideration. Two distances are chosen as most important; 40 cm. (16inches) as representing ordinary reading distance and 6 m. (20 feet) asequivalent to distant vision. Accordingly, test object A is normallyarranged at a distance of 40 cm., whereas the second distance isobtained by interposing a lens I0 (Figs. 4 and 5) of an optical powerthat causes the rays coming from source A to have the same vergence asthey would have if A was at a distance of 6 m. This lens must be veryaccurately hingedso that it is correctly centered when swung intoposition. Since an intermediate object distance, for example 75 cm., issometimes used as in the case of temporary prescriptions, provisions formeasuring at that distance or at any other desirable distance may beincorporated.

Most'lenses to be tested have power, both spherical and cylindrical foroblique rays, if not for paraxial rays (a small). Even a very smallamount of power causes the test object A to be so far out of focus thatit can not be distinctly seen nor accurately aligned. This circumstancecan be met by re-focusing the telescope until it furnishes a sharp imageof test Object A which can then be used for the accurate magnificationmeasurements above described. This is the only theoretically proper, andtherefore preferred way, to meet the difficulty of measuring themagnification of lenses which have also power. Since the object distancemay vary as above indicated, the telescope TA should have a focusingrange for object distances from well within 40 cm. to beyond 20 feet.The telescope adjustment is conveniently indicated on a drumhead scaleas shown at I| of Fig. 5, which may be calibrated in various ways to bediscussed hereinafter. If the object distance is changed by interposinga lens, as element l0 above described, the drum may have a double scale,as for example 40 cm. and 6 m. object distance, respectively.

The measuring of the dioptric properties of a lens, likewise an objectof the present invention, will now be described.

The power of the lens to be tested can be measured by means of afocusing telescope having a focusing range, and scale means covering along range of object distances as above described. When measuring thelens power in this manner, the object distance is fixed relatively tothe reference point of the instrument. The telescope is adjusted until asharp image is obtained, and

the power read on its scale.

This way of dioptric measurement reproduces rather accurately theconditions prevailing when an ametropic eye is corrected by anophthalmic lens. The difference between the telescope adjustments forobtaining a sharp image of the object with and without lens indicatesthe amount of ametropia which that lens would correct.

The telescope adjustment can be calibrated in different ways. If, withnite object distance, the power change is referred to the ocular lenssurface, it is defined as the reciprocal of the image distance from thatsurface and called vergence power. The power may also be defined as thedifference between the vergence of the light after passing the lens andits vergence before entering it, and is then usually called vergingpower. For infinite object distance, both vergence and verging power,referred to the ocular lens surface, become alike and are usually termedvertex power or back focal power. Instead of referring these powers tothe ocular lens surface, they can also be related to any other point onthe telescope axis.

By applying auxiliary lenses |03 (Fig. 5), the focal distance oftelescope TA can be changed tc comprise several measuring ranges withinwhich its focus can be gradually adjusted. In this manner, a telescopeof smaller range can be used to cover the entire measuring range. Disklill will then carry an appropriate number of scales, one for eachauxiliary lens.

The most accurate way of determining power characteristics is to measurethem as above described, with the aid of a focusing telescope.

Dioptric power may also be measured by changing the objectv distanceuntil the image appears sharp in the telescope which, in this instance.may be a fixed focus instrument. For

this purpose, object A can be put on a track B (Fig. 5) with suitablescales for measuring the object distance adjustment. An auxiliary lensI0 may again be used, and track B will then have two scales, one forreading position and one for distant vision, the first to be usedwithout, and the second with lens I0. This method of measuring power bymoving object A introduces an error since the object adjustment depends,for example when measuring vergence power, not only upon the power butalso the shape (that is properties not effecting power, as cupping andthickness) of the lens. However, this error is negligible for mostpractical purposes.

It should be noted that, for magnification measurements, the testobject, here light point A, must be placed at the correct objectdistance for which these measurements are to be made. For example, ifmeasurements for reading and infinite distances are to be taken with theinstrument herein described, object A will be actually placed a distanceof 40 cm. from the .reference point, and optically placed 6 m. from thatpoint.

Since, as is well known, the light path through an optical system isreversible, the present method may be carried out as indicated in Fig.4'L which is identical with Fig. 4 with the only difference that thelens to be tested and the test object are at diierent sides of referenceand pivot point N.

Apart from this reversal, the testing method is in this instance exactlythe same as explained with reference to Fig. 4, as will be understoodfrom a comparison of the two figures.

Corrective spectacle lenses are used at varying distances from the eyes,they may be tipped relatively to an eye, or decentered. In order toreproduce these conditions, the holder for the spectacle lens ispreferably mounted in such a manner that the lens can be adjusted bymeasurable amounts to assume any of the above positions relatively tothe elements representing the eye, as pupil P and pivot point N. 1

Since in certain cases two lenses are used in front of an eye, and sincethe positions of the lenses relatively `to each other and to the eye mayvary as above indicated, a second lens holder for a front lens may bearranged, permitting the same adjustments as the holder for the occularlens, namely variations of distance, tipping and decenteringindependently of, or together with, `the ocular spectacle lens.

In order to bring any particular meridian of the lens to be tested intoproper relation to the axes of the instrument, provisions are made forrotating the lens about the axis defined by telescope TA and point N.

Sc far, tests determining the lens effect on peripheral parts of thefield of vision, but only with a xed, straight ahead line of vision havebeen considered. Since the eyes are not always aligned with the opticalaxes of the spectacle lenses, especially in the instances of lookingdownwardly as when reading, and of corrections involving prism effects,the present invention includes measurements under such conditions. Inthe first instance, the eye in question looks at a point, outside of theoptical axis of the corrective lens, 'changing its position relativelyto the head by turning about its center of rotation. `In the otherinstance, the point of attention may or may not be in the optical axesof the lens unit; at any rate, the prism effects, if any, of all typesof lenses (including prisms proper and decentered lenses) involvemovements about the center of rotation, are frequently of greatimportance and must be accurately measured for central as well asmarginal portions of the lens.

In order to provide for measurements of this type, the present testingmethod provides relative rotation about a point' corresponding to thecenter of rotation of the eye, as indicated in Figs. l, l'A and lb and4, 4, 4h and 4.

These measurements, which constitute an important aspect of ourinvention, will now bedescribed more in detail. Referring to Fig. 1*,which is similar to Fig. 1 and has corresponding reference characters,the optical axis of lens La coincides with the visual axis U of eye E innormal position. If the eye is now rotated through angle' (for examplemeasured on scale S3 of Fig, 4), about center of rotation R, it assumesposition E1', indicated in dotted lines, and visual axis u and nodalpoint N move into positions ur and Nr, respectively. It is important toexamine the properties of the lens in the region around axis ur, forvariousv angles ar, as above explained. The signicance of rays vr, m",points Ur, Vr, Vr', and angles r and Aer will be understood withoutfurther explanation by comparing Figs. l and 12, bearing in mind thatcenter of rotation R and lens La remain fixed relatively to each otherwhereas the visual axis with the nodal point assumes a new positionrelatively to the lens, and that the lens portions surrounding axis urare now to be tested as previously explained with reference to Figs. 2to 4.

The testing instrument to be described in detail hereinafter reproducesthis situation as indicated in Fig. 4b. In this figure, L is the lensrotated at angle s about center of rotation R, with the new pseudo nodalpoint Nr fixed in the instrument. The measurements are then taken aroundpoint Nr as reference point, as described when discussing Fig. 4.

If prismatic lenses are measured, conditions are somewhat different, asfollows:

Figs. 1b and 4, which' are now referred to, show a at prism PE, but itwill be understood that any lens elements having prismatic effects areintended to be represented by this fiat prism. An

eye E looking at a point U is again shown. `When prism PE is placed infront of the eye, the latter must rotate through an angle in order tosee point U. It assumes then lposition Ep, and the new visual axis updetermines now that region of the lens whose properties are to bemeasured, by means of rays subtending with up angles a at the displacedpoint of reference Np.

In order to reproduce this condition in a lens Y testing instrument,according to Figs. 4 and 5, the prism must be rotated about a pointcorresponding to center of rotation R in the direction opposite to therotation of the eye shown in Fig. la. This adjustment through angle forexample about axes RV or RH of Fig. 5, is schematically indicated inFig. 4. It will, however, be evident that the ray of symmetry AS comingfrom object A does not coincide with the axis AI of the instrument, asindicated in Fig. 4c. Also, the region of the prism around ray AS whichis being examined will not be the same as that around ray up of Fig. 1b.It is, therefore, necessary to shift the prism to bring that portionthereof in the measuring range whose characteristics are to bedetermined. This is indicated at PE of Fig. 4. Such a shift may increasethe prism power of the lens and hence angle ,s must be corrected tocompensate for this change. By two such successive approximations, theproperties of lens elements having prism effects can be examined underconditions at which such lenses would be used in front of an eye.

Many spectacle lenses to be tested are designed to correct astigmaticdefects and also meridional sets of perpendicular axes, each of whichmay be inclined to the horizontal and vertical meridians.

For this purpose, provisions are made to carry out such measurementswithout removing the lens or the spectacle from its holder. The latteris mounted in such a manner that any lens can be rotated about the axisthrough points A and N. Provisions may further be made for measuringperpendicular meridians without rotating the lens. Accordingly, as shownin Fig. 5, the instrument may provide axes of rotation perpendicular tothose shown in Fig. 4. Assuming the latter to be in a horizontal plane,they correspond to horizontal axes NH and RH of Fig. 5, whereas NV andRV are vertical axes intersecting the horizontal axes at reference pointN and center of rotation R.

By setting one principal or eikonic meridian of the lens to be testedvertically or horizontally, measurements in the meridian perpendicularthereto can thus be made without resetting the cated for rotation abouta vertical axis, at STV and SIV.

By adding a rotatory adjustment of telescope T about the axis, or axes,through center of rotation R, the scales upon which magniilcations areread can be set to zero by initially considering the prism power,thereby eliminating the correction for that power, as will be discussedlater.

It will be understood that the various pivots must not necessarily bearranged as shown in Fig. 5, as long as the movements indicated in Fig.4 can be carried out. For example, it will be preferable in certaininstances to have two pivots through the center of rotation outside ofthose through the reference point, which facilitates the measuring oflens properties along oblique axes through the lens, as .whenreproducing the reading position or the measuring of the opticalproperties of prisms. In this respect, practical considerations willgovern the design of the instrument, and especially whether all possiblemovementsare provided in a large instrument of universal applicability,or whether only the essential movements are incorporated in a morecompact apparatus.

Particular attention must be given to the alignment of the test object,as for example light source 8, with'the cross hairs of the telescope. Ifthe lens system to be tested has only spherical surfaces, the testobject, focused as a sharp light point, can be easily aligned with theintersection of the cross hairs. If the lenses have cylindricalcomponents and the meridian in which the measurements are carried outcoincides with one of`theprincipal meridians of astigmatic correction,accurate alignment is possible by consecutively obtaining sharplyfocused astigmatic lines and aligning them with the respective crosshairs of the telescope.

If, however, the meridian in which the measuring light beam swings doesnot coincide with one comes very difficult unless special provisions aremade. It will be evident that any single sharply focused astigmatc line,if oblique to the cross hairs, can not be exactly aligned therewith. Oneway of eliminating this diiculty is to provide a rotatable cross hairpiece which permits the rotation of a cross hair into the direction ofany astigmatic line. Another more direct method is the following one.

Referring now to Fig. 23, T indicates the telescope tube, 26| theeyepiece and 262 the object glass. Between these elements of thetelescope are mounted the two halves 263 and 264 of an additional lens,263 being, for example, fixed to the tube and 264 adjustable along theoptical axis by means of separate adjusting means, for example, aconventional rack and pinion drive.

With the aid of a telescope of this type, one astigmatic line can besharply focused with element 263 byv adjusting the eyepiece, and theother line by separately adjusting element 264. The two lines will thenappear as a sharply focused cross whose point of intersection can beexactly aligned with the cross hairs 382 of the telescope, as indicatedin Fig. 24.

Fig. 25 illustrates another device for the same purpose. -T is again thetelescope, shown in its relation to reference point N and lens to betested L. Between T and N is an arrangement of two prisms 21| and 212with semi-reflecting light dividing surfaces 213, of a well known type,two reflectors 214, 215 and a lens system 216 therebetween. It will beevident that the light coming from object 8 is at 212 split into twoportions I and I' which are again superimposed at 21|. One sharpastigmatic line can be obtained by focusing beam I with the telescope,as usual, and the second line by adjusting lens system 216. The eect isthe same as above described with reference to Fig. 24.

Still another possible arrangement for aligning lenses with cylindricalelements will now be understood without detailed explanation. Instead ofusing lens halves 263 and 264 as shown in Fig. 23, two cylindricallenses may be introduced either instead of, or in addition to the halflenses, the axes of the cylinders forming a right angle and beingaligned with the axes of the lens to be tested. 1

Concerning the selection of a point of reference N, we are aware of thefact that the nodal point of the optical system spectacle lens-eye isdiierent from the nodal point of the eye alone and that the conceptnodal point is correct only for paraxial rays, the nodal points forvarious axes forming a caustic surface. However, the effect of thespectacle lens is a change of the object space, hence the rays enteringthe eye can be considered as coming from a changed space, bydisregarding the spectacle lens. Concerning the deviations of nodalpoints for other than paraxial rays, the power oi' the lenses to betested is comparatively low, so that the above mentioned caustic can bereplaced by a pseudo nodal point N which is the one herein referred toand whose position is indicated in Fig. 1. A

It must also be considered that the eyes perceive an object iield on theone hand by momentarily iixating a single prominent object, and on theother hand by slightly moving the eyes to change the center ofattention. In the first instance, that is regarding each fixation objectseparately, the ray pencils in question pass through the pupils of theeye. Considering the change of centers 0f attention, the light pencilsfor consecutively per'- ceived objects pass through the centers ofrotation of the eyes. Since the nodal point is located approximatelyhalf way between pupil and center of rotation, it is also the bestchoice for a reference point if its selection is considered in thisaspect.

It will now be evident that, if the pseudo nodal point is selected asreference point, .the total magniiication of the lens will be measured.It is, however, understood that the reference point can be shifted withrespect to the lens to be tested, and that the present invention is notlimited to the selection of any such point.

The diaphragm of the instrument which represents the pupil hasdimensions and a distance from point N corresponding to the actualdimensions of the eye.

The instrument which is schematically shown in Fig. 5, and the lenstesting method which can be carried out therewith will now be describedmore in detail, with reference to Figs. 5 to`22.

In these figures, I is a. rigid base, preferably a standard screwed tothe floor, and ending in a plate 2. Rigidly flanged to column I is anarm 3 extending laterally and supporting a point light A which may be asource of light behind a pin hole in a mask or, as in the presentexample, an electric bulb 6 in a housing 1', and, in front thereof, amicroscope objective 8. This objective images the lamp iilament as avery small star point of considerable light intensity, especiallysuitable for focusing it on the cross hairs oi a telescope, as hereindescribed, and constituting a test target.

In order to change the apparent distance of this test object, auxiliarylenses I0 are provided which are inserted in a disk |9 (Figs. 6, 7, 8)rotatably mounted on a rigid tube 20 xed to standard I at 29 and 30.Suitable stops (not shown) are provided for selectively inserting one ofthese lenses in front of the test target.

Rotatably resting on plate 2 is arranged a. standard disk I I, whoseangular position relatively to the standard can be determined and iixedas follows:

Plate 2 is provided with notches 9, whereas disk I I has an extension I2supporting a spring plunger I3 which, if released, engages one ofnotches 9 and, if retracted by means of knob i4 permits relativerotation of lplate 2 and disk In this manner, disk II can be placed inselected angular positions determined by the angles of notches 9. Asshown in Fig. 6, knob I4 constitutes a nut into which plunger I3 isthreaded; hence, by rotating the knob, the plunger can be permanentlywithdrawn from engagement'with the notches, leaving disk I I free torotate. A handle I5 on arm I6, fastened to disk is provided for thatpurpose. In order to x the disk relatively to the standard inany'desired position, the following arrangement is made.

Plate 2 and disk I I are provided with sectors 22 and 2 I, respectively(Figs. 7 and 8) which can be clamped together by means of an arrangementshown in Fig. 9. Sector 22 has a T-shaped groove 23 wherein slides abrake shoe 24 and a sleeve 25. A bolt 26 turns Within sleeve 25, havingat one end a thread 21 screwed into brake shoe 24, and at the other enda bevel gear 3|, consisting of a housing 32 xed to sleeve 25, a gearwheel 33 fast on bolt 26 and a second gear wheel 34 meshing with 33 andfastenedv to a shaft 35 extending through tube 36 to hand knob 31conveniently arranged near handle I5 (Fig. 7). Segment 2| has two eyes4| and 42 (Fig. 10) extending on either side of sleeve 25. Eye 4|supports a plunger hous- 75 ing 43 with plunger 44 and spring 45 tendingto push the plunger towards sleeve 25 and confining the latter betweenplunger 44 and rod 46 threaded into eye 42. A shaft 41 extends to knob48 arranged near brake knob 31 above described.

It will be evident that by turning knob 31, gears 33 and 34 and bolt 26are rotated, the latter moving brake shoe 24 towards or against segment22, thereby releasing disk |I from plate 2, or firmly connecting itthereto. By rotating knob 48, bolt 46 is screwed towards or away fromplunger 44, thereby moving sleeve 25 and with it disk In this manner,disk II and the structure supported thereon can be exactly adjustedrelatively to the base, and arrested in any position independently of.notch arrangement 9--I3 which denes certain predetermined angles. Twobearing standards 5| and 52 are fastened to disk as shown in Figs. 6, 7and 8. Journaled in bearings 53 and 54 is a rigid, balanced telescopeframe 56. Standard and frame 56 have segments 62 and 6|, respectively(Figs. 6 and 8) which are equipped with a brake and adjustment devicequite similar to that employed for the fine adjustment of disk I andshown in Figs. 9 and l0.

Brake screw and adjusting screw are controlled by shafts 63, 64 (Fig. 8)and knobs 65, 66 (Fig. 7) which, of course, move with frame 56, just asknobs 41 and 48 move with handle I5 and disk I I. In order to keepschematical Fig. 5 as simple as possible, the fine adjustment deviceshave been omitted therefrom.

Aligning telescope TA (compare Fig. 4) is mounted on frame 56 in thefollowing manner (Figs. 6, ll, 12 and 13). In order to provide a rigidsupport for this telescope, a platform 1| is directly fastened to frame56 at 12 (Fig. 11) and also connected thereto through brace 13 andcolumns 14, 15. A plate 16 with a ball support 16B rests on platform 1|and telescope flange 11 is screwed through 16 to platform 1I by means ofthree bolts 18 which permit tilting in any meridian of flange 11 aboutthe apex of ball 16e.

Flange 11 extends into pin 8| surrounded by a sleeve 82 which has twotelescope journal brackets 83 and 84, respectively. A collar 85 isfastened around sleeve 82 (Figs. 12 and 13) and has two prongs 86, 81with screws 88, 89 bearing against stop 9| of plate 11. By means of thisarrangement, the telescope can be adjusted about the axis defined by pin8|.

Telescope trunnions 92 and 93 rest in the journais of brackets 83, 84,mentioned above. A hoop 94 `with two adjusting screws 95, 96 is fastenedto trunnion 93, the two screws engaging stop 91 of sleeve 82 andpermitting adjustment of the telescope about the horizontal axis definedby trunnions 92, 93. Levels 98, 99 provided for aligning the telescopewith the optical axis of the instrument in a position which will bediscussed later.

Telescope TAis preferably a focusing system, l

` ing on a rigid tube |04 (Figs. 6 and 7) supported by cross pieces |05and |06 of frame 56. Also lfastened to disk |03 is a diaphragmconstituting pupil P (Fig. 18). f

Reading telescope 'I'R is likewise associated with frame 56. as follows.Plate III is secured to brace 13 with two columns I I2 and ||3 (Fig.11). Resting on plate |I| on a point support II 4 and pressedthereagainst by screws II5, II6 is a bracket plate ||1 (Figs. 6 and l1)with an adjusting screw I3 upon which rests the ocular end of telescopeTR. The other end is supported in ring I 2|,vmounted on tube |04, byfour wing screws |22.

Coming now to that part of the instrument which supports the lens to betested, a U-shaped outer yoke |3| is pivoted on a vertical pinion |32fastened to disk II. Attached to yoke |3|l is a plunger holder |33(Figs. 6 and 8) with plunger |34 and plunger knob |35, the plungerengaging notches |36 of a sector |31 fixed to disk By means of notches|36 and plunger |34, yoke 3| can be rotated on disk in the manner inwhich the latter is moved upon standard plate. 2 by means of plunger I3and notches 3.

Journalled in outer yoke |3| at |38 (see Figs. 8 and 14) is intermediateyoke I4I4, and similarly journalled, at |39, in intermediate yoke |4|but with a different axis of rotation, is inner yoke |5|. The swingingmovement of the three yokes relatively to one another can be regulatedas follows. An arc |42 (Figs. 8, l5, and 16) -is fixed'to intermediateyoke |4|, supporting in a slot a two armed lever |43 hinged at |44 andhaving at one end a roller |45 adapted to engage 'the notches |46 ofblock |41 secured to outer yoke |3|. At the other end of lever |43, aspring |48 presses roller |45 downwardly. A handleA I5Ia extends fromlever |43 through window |52, permitting convenient disengagement ofroller |45 from notches |46. A pivot support |53 (Figs. 8 and 16 to 19)is screwed to inner yoke |5| and extends into a clamp fork |54straddling arc |42 and having a clamp screw |55 for arresting relativemovement of intermediate yoke |4| and inner yoke |5| in journals |39.

Clamped to support |53 by means of a ring I 56 is pivot |51, Fig. 18, onwhich turns wing plate |58. Plate |58 has two wings |6| and |62 (Figs.1'1 and 19) sliding in slots of gussets 63 and |64 tted into the cornersof inner yoke |5| Wing |6| and gusset |63 are fitted with a scale |85and indicator |66, respectivelyfwhereas gus- .set |6`4 is provided with,clamping screw |61 by means of which wing plate |58 and the structurebuilt up thereon can be fixed relatively to the inner yoke |5|.

It should be noted that, corresponding with Fig. 4, pivots |39 and thecenter of rotation of wing plate 58 are in the same vertical plane,namely the plane through a point corresponding to the center of rotationof the eye, whereas horizontal pivots |36, 53, 54 and the vertical axesof rotation of outer yoke I3I, disk and standard are located in avertical plane through a point corresponding, for example, to the nodalpoint of the eye. These relations are clearly indicated in Fig. 5.

Fastened to wing plate |58 is a slide carrier |1| provided with athreaded spindle |12 which can be rotated in slot |13 by means of knob|14. A slide block |8I, Figs. 17, 18, has a tongue |82 extending intoslot |13 (Fig. 18) and having an inner thread engaging spindle |12. Byturning knob |14, the slide block can be moved along the'ax'ls of wingplate |58, and the relative position of lens L and reference point Nchanged within a wide range oi' distances between L and N.

Block |8| has a slot |83 extending at right angles to tongue |82 andwing plate slot |13, and supporting a lens holder plate |84 (Figs. 17,18, 19). By means of screw |85, plate |84 can be adjusted at rightangles to spindle |12. Plate |84 has a curved top |88 with arcuate slot|86 and arcuate guideway |81 whose centers of rotation are in the axisof telescope TA. A lens holder block |9| swings in guideway |81 and canbe adjusted relatively to plate |84 by means of screw |92 fastened inblock |9| and having a thumb screw |93.

Slidably fastened to block |9| by means of two rails |94, |95 (Figs. 17,18 and 20) is lens holder slide |96 having a threaded tongue |91 alignedwith boss |98 of block |9|. Screw spindle 20| is mounted on boss |98 byholding screw 202 and can be rotated with the aid of thumb disk 203,moving tongue |91 up and down on block |9|, along rails |94 and |95.Fastened to slide |96 and tongue |91 is lens holder or spectacle yoke200 which is built up on bridge 2|0. This bridge has two journal bosses2|| and 2|2 extending laterally therefrom. Supported in these bosses aretrunnions 2|3 and 2|4 of lower spectacle bar 2|5. Bar 2|5 has two postsupports 2|6 and 2|1 (Figs. 18 and 19) from which rise posts 22| and222. An upper spectacle bar 223 has two guide sleeves 225 and 226engaging posts 22| and 222. Two springs 221 and 228, slipped over theposts, tend to press bar 223 downwardly. As shown in Figs. 17 and 18, aspectacle with lens L, or a lens alone can be conveniently insertedbetween grooved lower and upper bars 2|5 and 223, respectively, andsecurely held in position.

It will be evident that, by means of spindle |12, plate |84, block |9|and slide |96, a lens can be brought into any possible positionrelatively to plate |58 which rotates around the vertical axis throughthe center of rotation. It will be understood that for the lens holderas built up on plate |58 different designs may be adoptedfso long asthey permit proper fixation and adjustment of the lens to be testedrelatively to themain axes of the instrument.

Provisions may also be made for swinging the spectacle holder about anaxis approximately tangent to vits lenses. As described above, the lowerbar 2|5 is journalled at 2|| and 2|2. As shown in Figs. 17 and 21, anangular arm 23| is attached to bar 2|5. A screw 232 with knob 233distances arm 23| from cam face 234 of bridge 2|0 against the force ofspring 235. Arm 23| has a pointer 236 which, with scale 231, permitsmeasured adjustment of the lens'holder, about axis 2| |2| 2, by turningknob 233.

Disks I9 and |03 (Figs. 6 and 7) for auxiliary lenses and pupil may bemounted with arresting devices, as for example shown in Fig. 22. In thisfigure, 34| is a cross piece (shown in Fig. 7) of frame 56. 342 is anotched wheel fixed to tube |04. An arrester 343 consisting of bellcrank 344 hinged at 345, spring 346 and roller 341 engages notches 348arranged at points corresponding to aligned positions of the lenses inholder |03. It will be evident that the auxiliary lenses for focusingtelescope TA can be easily brought into proper position, the opening inwheel 342 permitting free. passage of light rays along the optical 'axisof theinstrument.

It is important to check the alignment and centering of the surfaces ofthe lenses to be tested, especially since a large percentage thereofwill have one or more toric surfaces. For this purpose, the followingtesting method is preferably used.

Referring to Figs. 5 and 26, an electric lamp 35| is placed in front oftelescope T. This lamp is comparatively small (for example a cystoscopeor ophthalmoscope lamp) and covered by an opaque shield 38| having asmall pin hole 386 towards the lens in the optical axis.

Lamp and shield being very small, they do not appreciably interfere withthe operation of the telescope. Preferably they can be lowered to be outof the way after having served their purpose. When a lens L is placedbeyond reference point N. as indicated, and lamp 35| turned on, each ofthe surfaces of the lens acts as a mirror and forms an image of thepoint source of light. The telescope can then be focused on theseimages, their positions indicating the relative position of the centersof the surfaces. The lens may then be tipped or rotated and adjusted sothat both the images fall on the cross hairs of the telescope, provided,of course, that the lens has no prismatic effect. If the surfaces of thelens are torics, two images of the point source of light are derivedfrom the two principal meridians of each surface.

l These images are, of course, the astigmatic lines.

In a given doublet lens it is easy to detect whether or not these linesare parallel for the same meridian of the lens.

It is easy to check by this method whether the axes of the toricscoincide with the principal meridians of the lens. This or a similardevice can also be used wth appropriate scales on the lens holder (forexample, on disk |08 of Fig. 17) to measure the difference between theaxes of the two toric surfaces of a bitoric lens lwith crossedcylinders.

As mentioned above when referring to Figs. 23 to 25, the cross hairs ofthe telescope can be arranged for rotation about the optical axis of thetelescope. Such an arrangement is indicated in Fig. 26 where 382 is across hair, 383 its rotatable holder, 384 an indicatorattached to 383,and 385 a scale on which the amount of rotation can be read by means ofthe indicator. By apparently superimposing the cross hair on the variousreflected linear images of the pin hole in cover 38|, and reading therespective cross hair positions, the angles between the toric axes canbe determined. In order to make this measurement more accurate, the lensmay be turned over and the readings repeated.

The method of testing spectacle lenses according to the presentinvention by using the above described modification of the testinginstrument will now be described.

Assuming that the total magnification is to be measuredthe lens to betested, alone or in its frame, is placed in the spectacle holder andadjusted relatively to pupil P (see Figs. 5 and 18) spectively. If theastigmatic meridians are measured, their position can be determined, aspreviously described, with the use of the telescope and scale |0|.

The dioptric power both of the test image and the primary and secondaryastigmatic images are then measured as above described, by focus-

